Real Dielectric Function Research Articles

First-principles calculations of optical properties of Titanium nanochains

In this paper, optical properties of linear, dimerized, zigzag and ladder nanowires were reported using the full potential linearized augmented plane wave plus local orbital method (FLAPW+LO) and by incorporating the generalized gradient approximation (GGA). Also, the super cell approach, infinite and free standing periodic nanowires were applied. Optical properties such as dielectric function, energy loss function, optical conductivity and reflectivity of nanochains were investigated. Moreover, the results of nanochains were compared with the findings of the present author’s previous works on the bulk and nanowires (NWs). The properties, especially reflectivity and static dielectric function, changed significantly with configuration. The static dielectric function increased slightly from 1.9 in the dimerized nanowire to 3.06 in the linear one. The negative value of the real dielectric function occurred in some frequencies of the dimerized nanochains. Furthermore, equilibrium structure and inter-atomic distance were calculated, the results of which were in agreement with those of the previous works. In addition, nanowire reflectivity spectra showed a number of distinct peaks, which could be attributed to the plasmon resonance.

Ab Initio Calculations of Optical Properties of TlBr and TlCl Radiation Detectors

We report on ab initio calculations of the optical properties of TlBr and TlCl binary semiconductor compounds using the self-consistent scalar relativistic full potential linear augmented plane wave band method (FP-LAPW) within the local density approximation (LDA) including the generalized gradient approximation (GGA). The accurate calculations of linear optical function (refractive index, reflectance, coefficient of absorption, and both imaginary and real dielectric function) is performed in the photon energy range up to 20 eV. The predicted optical constants agree well with the available experimental data.

THz spectra study of MgF2 crystals

THz (terahertz) properties of MgF2 crystal (sample 1) and MgF2: Co crystal (sample 2) is measured using transmission THz time domain spectroscopy in the frequency range between 0.5 and 2.5 THz. The absorption coefficients α(ν) of sample 1 increases and has a maximum 24cm-1 with increasing frequency ν from 0.5 THz to 2.5 THz. The absorption coefficient of sample 2 is much larger than that of the sample 1, and its absorption band edge of the crystal lattice shifts to lower frequency side because of the dopant Co. Moreover, there is an absorption peak at 1.9THz and the absorption coefficient is greater than 70 cm-1, the force constant K of F-—Co2+ ionic chemical bond stretch vibration calculated from this peak is 3.40×10-2 N/cm. The THz spectroscopy can be an important technique for research of chemical bond in crystal. Real dielectric functions between 0.5 and 2.5 THz of the two samples are calculated by relations between optical constants. The ε1(ν) of sample 1 varies from 4.67 to 4.73 and ε1(ν) of sample 2 varies from 4.62 to 5.01.

General Properties of Local Plasmons in Metal Nanostructures

Under the quasistatic approximation, the characteristics of a local plasmon resonance of a metal nanostructure exhibit several general properties. The resonance frequency depends on the fraction of plasmon energy residing in the metal through the real dielectric function of the metal. For a given resonant frequency, the Q factor of the resonance is determined only by the complex dielectric function of the metal material, independent of the nanostructure form or the dielectric environment. A simple result describing the effect of optical gain on the Q factor is also obtained.

Gold and Silver Nanoparticles in Sensing and Imaging: Sensitivity of Plasmon Response to Size, Shape, and Metal Composition

Plasmonic metal nanoparticles have great potential for chemical and biological sensor applications, due to their sensitive spectral response to the local environment of the nanoparticle surface and ease of monitoring the light signal due to their strong scattering or absorption. In this work, we investigated the dependence of the sensitivity of the surface plasmon resonance (frequency and bandwidth) response to changes in their surrounding environment and the relative contribution of optical scattering to the total extinction, on the size and shape of nanorods and the type of metal, that is, Au vs Ag. Theoretical consideration on the surface plasmon resonance condition revealed that the spectral sensitivity, defined as the relative shift in resonance wavelength with respect to the refractive index change of surrounding materials, has two controlling factors: first the bulk plasma wavelength, a property dependent on the metal type, and second on the aspect ratio of the nanorods which is a geometrical parameter. It is found that the sensitivity is linearly proportional to both these factors. To quantitatively examine the dependence of the spectral sensitivity on the nanorod metal composition and the aspect ratio, the discrete dipole approximation method was used for the calculation of optical spectra of Ag-Au alloy metal nanorods as a function of Ag concentration. It is observed that the sensitivity does not depend on the type of the metal but depends largely on the aspect ratio of nanorods. The direct dependence of the sensitivity on the aspect ratio becomes more prominent as the size of nanorods becomes larger. However, the use of larger nanoparticles may induce an excessive broadening of the resonance spectrum due to an increase in the contribution of multipolar excitations. This restricts the sensing resolution. The insensitivity of the plasmon response to the metal composition is attributable to the fact that the bulk plasma frequency of the metal, which determines the spectral dispersion of the real dielectric function of metals and the surface plasmon resonance condition, has a similar value for the noble metals. On the other hand, nanorods with higher Ag concentration show a great enhancement in magnitude and sharpness of the plasmon resonance band, which gives better sensing resolution despite similar plasmon response. Furthermore, Ag nanorods have an additional advantage as better scatterers compared with Au nanorods of the same size.

Relativistic energy loss and induced photon emission in the interaction of a dielectric sphere with an external electron beam

An analytical expression for the energy loss suffered by a fast electron passing near a homogeneous dielectric sphere is derived within a fully relativistic approach. The sphere is described by a frequency-dependent dielectric function. The electromagnetic field induced by the passage of the electron is then obtained by expressing the solution of Maxwell’s equations for this geometry in terms of the scattering of the multipole expansion of the incoming electromagnetic field at the sphere. The energy loss is derived from the induced field acting back on the electron. The variation of the energy-loss spectra with both the radius of the sphere and the impact parameter of the electron trajectory is studied in detail. Part of the energy loss is transformed into radiation, which is also investigated. For spheres characterized by real dielectric functions, like those of ionic materials in the transparency-frequency region, it is shown that the entire energy loss is transformed into radiation. Examples of loss spectra and radiation emission spectra are given for a material described by a Drude-like dielectric function ~e.g., Al! and for SiO2 . @S0163-1829~99!12103-9#

Many-body effects at metal-semiconductor junctions. I. Surface plasmons and the electron-electron screened interaction

The electron-electron screened interaction near the metal-semiconductor junction is calculated analytically by the introduction of model bulk dielectric functions which contain all of the essential energy and wavelength dependent features of the real dielectric functions of the metal and semiconductor. The poles of the interaction give the dispersion relations for the surface plasmon excitations at the junction and these are studied for various parameters of the two bulk media. It is shown that there are two possible plasmon modes characteristic of bimetal and metal-classical dielectric interfaces. Also, the role of surface plasmons in Schottky barriers is discussed.